Atomic and electronic structure of polymer organic semiconductors: P3HT, PQT, and PBTTT

First-principles pseudopotential density functional calculations were employed to investigate the atomic structure of three organic semiconductors: poly[5,5'-bis(3-alkyl-2-thienyl)-2,2'-bithiophene] (PQT), poly[2,5-bis(3-alkylthiophen-2-yl)thieno(3,2-b)thiophene] (PBTTT), and poly(3-hexylthiophene) (P3HT). The calculations show that a substantial rotation of the conjugated planes around the polymer axis is energetically favorable for all three crystals. This rotation reduces the overlap of molecular orbitals, and therefore increases the effective mass in the pi-pi stacking direction. This impacts the mobility, which is estimated within an acoustic deformation potential model. Similar values for the effective mass and mobility were obtained for P3HT and PBTTT. Therefore, the higher mobility observed experimentally for PBTTT in comparison to P3HT could be a result of improved structural ordering rather than being an intrinsic property of crystalline regions of the polymer. Calculations indicate that substantial interdigitation of the alkyl side chains is energetically favorable for PBTTT.